1. Field of the Invention
This invention relates generally to membrane switches, and more particularly to a membrane switch which provides an actuation force which can be varied, prevents a vacuum from forming between the switch layers, does not require a hard mounting surface for actuation and can be utilized in an air bag assembly for an automobile.
2. Description of the Related Art
Membrane switches typically include two opposing surfaces, each of which include circuits thereon which, when engaged, complete the desired electrical circuit. To maintain the two opposing surfaces apart and the circuit in an open state, some type of insulating layer or spacer is disposed between the two substrates.
An example of such a membrane switch is illustrated in U.S. Pat. No. 4,382,165 which discloses two circuit sheets, one of which is adhered to a backer or stiffener board. Each sheet includes circuit patterns on opposing surfaces including circuit paths and circular contact pads thereon. An extremely thin nonconductive adhesive layer is applied between the circuit patterns which covers the circuit paths but includes apertures at the position of each circular contact pad. In order to prevent unintentional short circuiting from tolerance variations or sagging of the circuit sheets, a discontinuous layer of adhesive in the form of lines or dots is applied in the apertured areas.
Such a switch, however, must be utilized with a backer board and cannot provide variable actuation forces since wide spacing between the lines or dots causes sagging while small spacing can prevent ready actuation of the switch. Additionally, since the adhesive provides a seal about the apertured areas, a vacuum can be created when the switch is depressed preventing release of the contacts.
Another membrane switch is illustrated in U.S. Pat. No. 4,602,135 which discloses two sheets having opposing surfaces where one surface includes switch leads and the other surface includes circuit completing material. A nonconductive spacer is positioned between the switch leads and the circuit completing material where the spacer is formed as a grid made up of squares.
As with the above described patent, a vacuum is frequently created between the layers of the switch when depressed which prevents release and breaking of the circuit. Additionally, such a switch also requires a stiff backing or mounting surface to enable activation.
It therefore would be desireable to provide a membrane switch having an activation force that can be varied across the surface of the switch, or from one application to another, which prevents a vacuum from forming between the switch layers, does not require a hard mounting surface for activation, and can be utilized in an automotive air bag assembly without injuring the user or damaging the air bag before or during deployment.